Travelling wave magnetrons



April 19, 1960 R. G. RoBER'rsHAw ETAL 2,933,643

TRAVELLING WAVE MAGNETRCNS Filed March 24, 1955 2 sheets-sneer 1 F403l f Wam Funes-r 4u swing R. G. RoBER-rsHAw EVAL 2,933,643

'rRAvELLING wma MAGNETRoNs April 19, 1960 2 Sheets-Sheet 2 Filed March 24, 1955 FIG maven-mns Rose-nr Gssofv Foix-mwa, VILL/q Fave-51- Ml-Lswnv HTTORNEY United States Patent Oliice.

2,933,643 ,y patented TApr. Y1,9, 1aed Y TRAVELLING WAVE MAGNETRONS Robert Gibson Robertshaw, Wembley, and William Ernest Willshaw, Kenton, England, assgnors to Thel I This invention relates to travelling wave magnetron oscillator devices of the kind having an anode structure deining an even plurality of cavity resonatorsspaced round, and opening on to, a cylindrical electron orbit space within which is located the cathode of the magnetron.

Such devices are arranged to co-operate with means for producing a magnetic eld directed parallel to the axis of the electron orbit space, which iield, in conjunction with a radial electrostatic eld applied between the cathode and the anode, causes the electrons emitted from the cathode to ow round the cathode and excite the cavity resonators; the oscillating electric elds set up between the cavity resonators in the electron orbit space cause the electrons to become focussed or hunched into spoke-like regions which travel round the cathode in synchronism with a component of the electric eld set up by the resonators, so that there is a continual transfer of energy from the electrons to the electric iield.

Such devices have the advantage that they can be designed to produce oscillations of short wavelengths, for example l centimetres, at relatively high power and with relatively high eeiencies as compared with the conventional grid-controlled thermionic valve oscillators, and can also be designed to operate with reasonable eiliciency at much shorter wavelengths, for example 3 centimetres or less, which are outside the range of operation of the conventional thermionic valve oscillators.

The travelling wave magnetron devices also compare favourably with other forms of electronic oscillators, for example velocity modulated klystrons, in respect of eciency at these very short wavelengths, but for continuous wave operation at low powers, with low anode/cathode voltages and magnetic elds as used, for example, in radio receivers or portable transmitters, the mechanical difficulties of making the devices for operation at wavelengths of much less than 3 cms. increase rapidly with decreasing wavelength since the dimensions of the device, and in particular of the anode structure, become very small and the permissible tolerances in manufacture also decrease.

It has been proposed to reduce this dificulty by causing a magnetron of the type specified to operate on a spaceharmonic of its fundamental 1rmode frequency, and in order that the present invention may be fully understood, the nature of this proposal will be explained in more detail.

Magnetrons of the type specified are capable of operating in diierent modes of oscillation of the anode system which are characterised by dilerent values of the instantaneous phase difference between the electric fields in adjacent resonators. Thus if there are N resonators the phase difference between adjacent resonators may assume any one of the values 21m/N radians, Where n is one of the integers 0, l, 2 N/2. A value of n equal to N/2 corresponds to oscillation of the anode system in the so-called 1r-mode, the phase difference benents, which particular ones will hereinafter. be

tween adjacent resonators then being 1r radians, and this is the most usual mode for operation of. magnetrons of the type specified. v

The effect of the oscillating electric lields in th resonators is'to impose potential variations between'the adjacent anode segments between the .resonators and these potential variations may be regarded as a series, of potential waves Vtravelling round the anode in eitherV direction with angular velocity 21m/Mz radiansper sec,-` ond, where A is the free space wavelength of theelectrical oscillations in the resonators of the anode system, c is.- the velocity of the electrical oscillations in free space;I and the value of n corresponds to the particular ,rhodevr in which the resonators ofthe system are oscillating,y An electron travelling round the anode with angular ve- ,y locity 21m/An in the same direction as one of said waves in such phase as to transfer energy thereto will thus be maintained in synchronism with the wave and will con-' tinually transfer energy thereto. However it can be shown that Velectrons of certainother angular velocities will also be maintained in synchronism with the wave, and if the Wave, which is non-sinusoidal, be regarded asl formed by a series of Fourier components, these electron. velocities correspond to particular ones of .these comporeferred to as space harmonics. y j Thus if the wavelength of the electrical oscillations isthe space harmonics are those components whichv leiectively travel round the anode system with angular velocity 21m/kk, where k (defined-hereby) is an integer y which is different for dilerent space harmonics.

For the normal symmetrical arrangement of resonators,A k=n|pN and p=0,i1, i2 etc. For the purposes of this specification only positive values of p need be considered. The iirstmvalue in the series corresponds, off course, to the fundamental oscillation. It will later be` pointed out that with an unsymmetrical arrangement k may have other values in addition to those indicated by` the above formula. l

It will therefore be seen that it is in theory possible to operate a symmetrical magnetron of the type specified: in any particular mode, corresponding to a particular value of Iz, under any one of anumber of dilerent conditions giving diierent electron velocities round the cathode.

The manner in which this possibility-might simplify the aforesaid difliculty of designing magnetrons of the type specified for operation at high frequency will now be indicated. Thus the operating conditions of such a magnetron may be expressed to a rst degree of approximation byv the formula Y 0.9482v r ,2 V- k T Bll QJ] :(1) where The wavelength A of the electrical oscillations is, for operation in a particular mode, such as the ar-mode, and ignoring frequency changes which might result from the applied between anode and i loading of the device, determined by the dimensions-ofthe cavity resonators. Thus if, starting with a given design for which condition (l) is fulfilled, the resonators are modiiied to decrease A, then for the new design one or a combination of the following further modifications Will need to be made order that condition (l) may,l

' operation on a space harmonic in eration at relatively low power.

Y eration; V(b) involves reducing the dimensions of the anode system,` which gives rise to thejdithculties aforesaid,.o:r increasing the cathode diameter, whichV is undesirable, and it would appear that these diiculties might be avoided by means of (c).

One method of increasing k is by increasing the numi ber N of resonatorsin the anodeY system; by'this means the desired operating wavelength can in some cases be seemed without undue alteration Vof r.I and rcrand with-Y out Vundue increase of V or reduction of B, but further mechanical difficulties then arise, at very short wave.

lengths, in forming .the resonators with the required accuracy,.especially in view of the short gap fwidth at Vthe electron orbit space which is imposed bytheY larger numbei of resonators. t f

An ternative method is by increasing the Vvalue of p, Ythat is to say, causing the magnetron to operate on one of the space harmonicsY of the'anode potential wave, but this' method does not Y appear to have been successfully adopted hitherto.

We believe that this has been due to the fact that tbe design of the known magnetrons with which operation on a space harmonic has been attempted has not been suitablefor this purpose, Yand in particular the amplitude of the space harmonic of the anode potential wave with which interaction has been sought has, becauseV ofgthe way in which the device was constructed, usually been too small to permit such interaction. The main objectf of the present invention is to provide a form Yof magnetron of the type specified which is more suitable for continuous wave op- It will, however, be appreciated that valves in accordance with the invention can also be used for pulse operation Vand may in some cases be designed specifically for'such use.

The foregoing explanation has, for simplicity, been based on the assumption that thev anode resonators are symmetrically arranged around the cathode; such a symmetrical arrangement is not, however, essential for the construction of a magnetron in accordance with the invention and before statingpthe invention formally the more Ygeneral arrangement of resonators possible in accordance with it will first be defined.

Y'Ihe operation of an N-segment magnetron on'a space harmonic corresponds, as regards the number of effective antinodes in the instantaneous anode potential wave, with Ythe fundamental r-mode operation ofv an anode system having 2k resonators, where the value of k is related to N and depends on the particular harmonic considered, as aforesaid. Now imagine in a magnetron having N resonators, the circumference of the electron orbit space divided by 2k equally spaced points. Then with` a symmetrical arrangement of the resonators, the centres of the gaps at which the resonators open on to 'theV electron orbit space will each coincide ywith one Yof said points, which may be called a resonator point, and

of increasingthe value of k` pair.

For example in a 6 resonator system operating on the harmonic for which k is l2, there are 24 of said points and the resonators may be centredon the lst, 4th, 9th, 12th, 17th, and 20th points, starting at any one of theV points. The possible positions of the resonators Vcorresponding to thisY more general arrangement together with thesymmetrical arrangement Will hereinafter be referred to as the harmonic positions.

It may be noted that with unsymmetrical harmonic positions a greater Vnumber of space harmonics are available for interaction than with a symmetrical arrangement. Thus with -fr-mode operation of N resonators arranged non-symmetrically in N/2V pairsV as aforesaid, interactionV may be obtained with space harmonics corresponding-to values of k equal to pN/ 2, where p is any integer, whereas in the-symmetrical case the space harmonies with Ywhich interaction can Vbe obtained are those givenby k=N(p-}1/2). InV each series the firsty value corresponds, of course, to the fundamental oscillation.

ForV example, if N =6 then with the general arrangement aforesaid the possible values4 of k are 3.1?, that is tosay,3,6,9, l2 etc.

Y On the other hand with the symmetrical arrangement,

again for N :,6, the possible values ofV k-are 6(p+1/z), that isto say,l3,9, l5 etc.,

The invention can now be stated.

According to the invention a travelling wave magne` tron oscillator of the type specified has an anode substantially in the form of a -metal plate pierced perpendicular to lits major faces by an raperture Whose perimeter provides a cylindrical inner hole which forms the electron orbit space, theY diameter of this space being not greater than )t/ 4 and the axial length of the space being not greater than Sit/l0; the perimeter of said aperture also providesV an even plurality N, not exceeding six, of spaces which form similar cavity resonators extending radially from the electron orbit space and spaced round its axis in harmonic positions as hereinbefore dened; Va cylindrical cathode is mounted within Vthe electron orbit space so as to lie accurately coaxial there-` with; the anode and cathode structures are mounted within a sealed evacuated envelope'which is such that oscillatory energy from the anode. system can be radiated directly through the envelope; leads to Ithe anode and cathode are sealed through the envelope; andthe arrangement is such that inl conjunction with a suitable is not greater than M10 and the axial length Vof the space is also not greater than M 10.

The ratio Vof cathode diameter to electron orbit space -is not verylcriticalV in a valve in accordance with the invention, but with a valve designed specifically for continuous wave operation the value of the ratio is preferably arranged to be less than that normally used hitherto in comparable known valves,

' and a value of about 0.3 for the ratio is suitable. For

these resonator points will includey between each other the same number of other points'.

The more general arrangement of the resonators possible in magnetrons in accordance with the invention is Y Veven number, which is not zero, of said points included between the adjacent resonator points in different pairs,V

the same number of points being included between eachvalves in Yaccordance with the invention designed for Apulse operation the ratio may be about the same as that normallyY used'hitherto, that is to say Vabout 0.6.

f Preferably the` angular circumferential width of the gaps where the resonators open on to the electron orbit space is not less than fr/Zk radians. It may be noted that the theoretically maximum possible gap width obtainable in the Zk-resonator magnetron, assuming iniinitely thin anode segments, is 1r/k radians; the gap width lin a magnetron in accordance with the invention may, however, be appreciably greater than that for the Zk-resonator magnetron, which has the advantageof enabling theV amplitude of the desired space harmonic of the diameter of the.

the anode potentialjwave to be increased relativelyto interfering components, thereby facilitating excitation of this space harmonic. However, for certain values of the gap width, the amplitude of a space harmonic may be zero; thus the amplitude of the space harmonic k is zero for values of equal to 360/k, where 0 is the angle subtended by the gap at the axis of the electron orbit space, and these identical values must be avoided as far as the wanted space harmonic is concerned. On the other hand, use may in some cases be made of this property by arranging the gap width to correspond to the critical value of ran unwanted space harmonic, and thereby preventing excitation of that harmonrc.

These last mentioned properties depend on the anode potential wave round the anode surface being of the nature of a step-function, that is to say zero along the surfaces of the anode segments and rising infinitely steeply at the edges of the resonator gaps. In practice there is invariably some rounding of the edges of the resonator gaps; now one of the advantages of a magnetron in accordance with the invention, resulting from the simple resonator system and relatively wide gaps, is that the anode can often be formed by the relatively simple hot extrusion pressing process known in the art as hobbing, which in some cases may accentuate the rounding of the gap edges. To minimise this effect the sides of a resonator slot may be inclined away from the perpendicular to the chordal plane at which the slot intersects the electron orbit space, so as effectively to sharpen the edges of the gap formed by the intersection, this angle of inclination preferably lying between zero and (9D-p), where is angle between the chordal plane and the tangent plane at the edge of the gap. it will be appreciated that for practical reasons of convenience in manufacture, such inclination of the sides of the slot may start slightly away from the mouth of the slot. .This inclination of the sides also has the eect of increasing the L/ C ratio for the resonators and with it the strength of the electric field at the resonator gaps, and hence the valve eflciency.

With a magnetron in accordance with the invention, especially when the N resonators are arranged symmetrically around the axis of the electron orbit space, as is preferred, we have found that stable operation on a space harmonic of the rr-mode can readily be obtained. We believe that the reason for this is that the limitation of the number of resonators to less than 6 reduces the number of possible interfering modes and their components in the anode potential wave, and the dimensional limitations stipulated ensure the presence of relatively strong space harmonics in the anode potential wave, as well as minimising the occurrence of interfering components. 'Ihe provision for the output of the magnetron to be radiated directly through the envelope avoids the necessity for an output coupling loop within one of the cavity resonators and prevents disturbances arising from that cause; however, in connection with this last mentioned point it may be possible and advantageous in some cases to provide a short probe projecting perpendicular to the anode plate from one of the anode segments for increasing the coupling of the anode system to a waveguide in one direction; usually, however, such a probe will not be necessary and isrthen preferably not used in order to minimise the disturbances introduced into the anode system. Thus preferably at least one of the cavity resonators extends so close to. an edge of the plate that said edge part of the plate constitutes in effect an output loop the electromagnetic eld linked with which in operation of the device extends to the exterior of the envelope.

Preferably the anode and cathode structures are arranged within a closely tting cylindrical glass envelope dimensioned to t snugly withina waveguide adapted to transmit oscillations of the output frequency of the magnetron,- and for blocking the passage of high frequency energy in the reverse direction along the waveguide, the

anode plate is mounted at one end on a cylindrical metal:

sleeve which approachesV closely and coaxially the inner surface of the glass envelope and is of axial length electrically equivalent to one quarter of a wavelength at the output frequency of the magnetron, so as to be capable of forming with the wall of the waveguide, when the device is inserted therein, a choke which presents a high impedance to the high frequency energy.

With the magnetron in accordance with the invention having six resonators, for which N =6 and n=3 for oscillation of the anode system in the vr-mode, the permissible values of k for space harmonic operation are 9, l5, 21, etc., and preferably the magnetron is arranged to operate on the rst space harmonic, corresponding to With a magnetron in accordance with the invention having four resonators, for which N =4 and n=2 for oscillation of the anode system in ther-mode, the permissi ble Values of k for space harmonic operation are 6, 10, 14 etc., and preferably the magnetron is arranged to operate on the rst space harmonic, corresponding to k,=6.

With a magnetron in accordance with the invention having two resonators, for which N=2 and n=1 for oscillation of the anode system in the 1r-mode', the permissible values of k for space harmonic operation are 3, 5, 7 and the magnetron is preferably arranged to operate on the second space harmonic, corresponding to k=5.

A particular advantage obtainable With a magnetron in accordance with the invention is that its simple anode structure renders it readily tunable over an appreciable frequency range by means of adjustable reactive loads without introducing such interfering components in the anode potential wave as to give rise to mode change troubles or appreciable loss of eciency of operation on the desired space harmonic.

The invention will be further described with reference to the accompanying drawings, in which Figures l, 2 and 3, illustrate two forms of magnetron in accordance with the invention, Figures 4, 5 and 6 indicate how magnetrons in accordance with the invention may be used, and Figures 7 and 8 illustrate modifications which may be incorporated in a magnetron in accordance with the invention.

One embodiment of the invention will now be described by Way of example with reference to Figures l and 2 of the accompanying drawing, which show two side views of the magnetron taken at right angles to each other.

The magnetron comprises a cylindrical glass envelope 1 closed by a pressed glass base 2 which carries seven rigid terminal pins 3 on its outer side and through which are sealed electrical supply leads terminating in seven lead-in and electrode support wires 4 within the envelope.

The electrode system is mounted on these support wires and includes an anode in the plate 5 screwed to the outer side of the base of a cylindrical copper choke cup 6 with an edge of the plate extending along a diameter of the base; the screws 7 which attach the anode in the cross-bar of a T-shaped nickel support member 8, the down stem of which lies along the axis of the cup and is welded on each side to two of the support wires 4 so that the cup and anode are supported thereby substantially co-axially within the cylindrical envelope 1.

The anode plate S is pierced perpendicularly from the form of a rectangular copper` 5 to the cup 6 pass also through holesA centre of one major face to the centre of the other by an aperture Whose periphery provides a circular space 9 and four spaces 10 each in the form of a sector of a circle, the spaces 19 being symmetrically arranged around the space 9 so as to dene between them four substantially rectangular anode segments 11 projecting along two lmutually perpendicular diameters of a circular hole coaxial with the space 9, said diameters being each inclined at an angle of 45 to the axis of the choke cup 6. The electrode system also includes an indirectly heated cathode`- 12 supported accurately coaxial with and within the elecfv` tron orbit space 9 between two inica cathode support faces of the anode plate 5.V

Each vmica sheet is provided near one edge with two holes,y into each` of which is' lixed a nickel eyelet 14," and each mica sheet is attached to the anode plate by two copper-nickel pins which pass through the nickel eye-4 lets and through corresponding holes pierced through the anode plate adjacent to the choke cup 6, the mica plates being clamped to the plate by nickel retaining sleeves 16 fitted overA the ends of the pins 15 and spot-welded to them; the mica sheets are spaced from the surfaces of the anode plate 5 by the heads 17 of the nickel eyelets 14.

At the region opposite the electron orbit space 9, each mica sheet 13 is pierced by four holes into which iit tags of a small, approximately rectangular, nickel cathode end shield 1S, the latter each lying on the side of the mica nearer the anode plate and being held in position by the bending over of the tags on the other side of the mica. The end shields 18 are coated with a material, such as titanium dioxide furnaced` in dry hydrogen to a tine black deposit', for increasing their thermal emissivity.

. Each end shield 18 carries a central hole which lies opposite-a hole pierced through the corresponding mica sheet, and through each of'these further holespasses one end of the cathode 12, which consists of a hollow nickel tube coated with electron emissive material over the part of its outer surface within the electron orbit space 9 and carrying a heater 19 Within the tube. Y One end of the cathode 12 and the corresponding end of the heater 18 are connected to a lead Wire 20 which passes, through a ceramic bushing 21 in the 'base of the cup 6, to one of the lead-in wires 4, and the other end of the heater is connected through a lead wire 22, which passes through a further ceramic bushing 23 in the base of the cup 2, to a different one of the lead-in Wires 4.

, The lead wires and 22 are each provided with a single turn loop for providing resilience and allowing for thermal expansion of the leads in use of the magnetron.

The envelope 1 is evacuated and sealed off at the end of the envelope opposite to the base 2, and is gettered by means of a getter element 24, which closes a wire loop carried by the remaining one of the lead-in wires 4, the loop being eddy current heated, after evacuation and sealing of the envelope, for dispersal of the getter.

The glass base 2 of the envelope is provided on its inside surface with a coating of magnesium oxide, applied in aqueous suspension anddried during the baking of theV envelope during the processing of the device, for increasing the mutual insulation between the terminal pins.

- The cathode support assembly is used for ensuring the accurate centering of the cathode Yduring manufacture in the following manner:

The holes through the anode plate 5 into which the pins 15 tit are, when formed, accurately positioned relative to the axis of the electron orbit hole 9, and the pins 15 are an accurate tit Within these holes, being staked and clenched into position using a circular tool. The plate 5 is mounted in a jig with the pins 15 in position in the anode plate and With a locating piece fitted into the'electron orbit space 9, said locating piece having an axially.

cylindrical extension of the same diameter as the cathode tube which projects from one side of the plate; over this side of the anode plate is tted the corresponding mica sheetV 13 with the end plate 1SV and nickel eyelets 14 tted,

therein but not clenched to the mica; the central hole in:

the Vend plate 18 is iitted over the extension of the locating piece and the tags of the end plate turned down to hold it iirmly Yin position on the mica; the nickel eyelets 14 are then pressed down to clench them iirmly to the mica. The mica sheet is then withdrawn from the pins 15, the anode plate reversed in the jig, and the procedure repeated for the other mica sheet on the opposite side of the anode plate; The locating piece is then withdrawn from the electron orbit hole, and replaced by the cathode; one end of whichV is inserted through theY hole in the end plate of the mica sheet fitted to the anode plate, after which the second mica sheet is'itted to the anode plate, over the pins 15, so that the other end of the cathode passes through the Vcentral hole" in the end `plate carried by this second mica sheet. -The two mica sheets are'then fixed in position by spot-welding the nickel retaining Y sleeves 16 on to the projecting ends of the pins 15.

The anodeecathode assembly is thereafter secured to the choke cup 6 and supporting strip 8, which is in turn mounted on its support wire V4,` andthe cathode and heater connections to the leads 20 'and 22 are then completed. Y Y

The magnetron is arranged fory the direct radiation of high frequency energy through the walls of the enevlope.

In one magnetron manufactured as described above, the followingdimensions of the various parts were found suitable for operation at a wave length of 3 centimetres, the valve operating in the vr-mode Von the rstspace harmonic' Y Millimetres Length of anode plate 5 along axis of envelope 13.1 Width of anode platerS 12.7 Thickness of anode plate 5 Y 25 Diameter of electron orbit space 9 2.0 Radial length of segments 11 8.0 Circumferential width of segments 11 l Diameter of cathode 12 0.63 Diameter of Vchoke cup 6 15 Axial length of choke cup 6 .Y-- 8 Diameter of envelope 1 119 1Outside. f ,Y

With a magnetic field of 2000 gauss applied in the di-V rection Vof the axis of the electron orbit space 9, the

Y anode/ cathode voltage required for operation on the first formed by the parts of the anode plate bounding the resonator 1li furthest from the choke cup 6.

It will be appreciated that minor modifications of the construction just described may readily be made YWithout appreciably affecting its suitability for operation on a space harmonic; for example the system of resonators 1 19 could be rotated through 45 and a short coupling probe provided on the anode segment furthest from the choke cup 6, said probe extending perpendicular to the plane of the anode plate. Y

. In the second embodiment of the invention the construction of the magnetron is .thesarne as that'shown in Figures 1 and 2 except that the anode plate 5 is replaced by the anode plate 25 shown in Figure 3, which represents a view of one of the major faces of the plate.

T he plate 25 is substantially rectangular and is pierced by an aperture whose periphery provides a central electron orbit vspace 26 and two resonator spaces each of which has Vthe shape of a segment of a circle 27 communicating with the electron orbit space 26 through a parallelsided slot 28 extending perpendicularly from the centre of the chord of the segment, said chords being parallel t0 each other'. It is important that the sides Yof the slots 28 should be dat and not lost by roundings of the corners at the ends of the slots, although for increasing the L/ C ratio in some cases the corners of the slots where they intersect the chords may be cut olf by planes extending perpendicular to the major faces of the plate, for example at an angle of 45 to the chords.

In one magnetron having an anodeY plate of the kind shown in Figure 3 and otherwise constructed as described g with reference to Figures 1 and 2, the plate 25 being mounted on the choke cups with the anode segments 29 (formed between the two resonators 27, 28) parallel to the base of the choke cup, the following dimensions of the anode plate were found suitable for operation at a wavelength of 3 centimetres, the valve operating in the :r-mode on the second space harmonic (k=5).

-Millimetres Length of anode plate 25 along axis of envelope 13.0 Width of anode plate 25 12.7 Thickness of anode plate 2.5 Diameter of electron orbit space 26 2 Radial length of slots 28 0.5 Width of slots 28 0.5 Radius of curvature of segments 27 3.7 Length of chords of segments 27 6.5 Diameter of cathode 0.6 Diameter of choke cup Axial length of choke cup 8 Diameter of envelope 119 l Outside.

With a magnetic field of 2500 gauss applied in the direction of the axis of the electron orbit space 9, the anode/cathode voltage required for operation on the second space harmonic was found to be 900 volts, and with an anode/ cathode current of 6 milliamps, the output ofthe device so operated was found to be about 500 milliwatts.

With the orientation of the resonators shown in Figure 3, a magnetron as just described is suitable for mag-- shaped supporting strip 8 is replaced by a hollow tube of non-magnetic material, such as a copper-nickel alloy, anged at one end where it is screwed to the inside of the base of the choke cup 6 and the anode plate 5, and secured to the four support wires 4 at its other end, which end is open or perforated to permit evacuation and degassing during the pumping of the device; in addition a mica sheet is sandwiched between the base of the cup 6 and the anode plate 5, the edges of the sheet extending into abutment with the inner surface of the glass envelope 1; the mica sheet is cut away where the anode plate 5 is screwed to the choke cup 6, and the base of the anode plate extends at these regions into abutment with the choke cup to permit good thermal and electrical contact between the anode plate and choke cup; in addition the lead wires and 22 pass through holes in the mica sheet, which serves also as an insulator replacing the ceramic bushings 21 and 23. A second mica sheet for supporting the free end of the anode plate from the envelope may also be provided in some cases Figures 4 and 5 illustrate schematically the way in which a magnetron of the form described with reference to Figure 3 may be utilised; in both gures the magnetron is represented only by the anode plate 25 and choke cup 6 within its envelope 1, the magnetron being shown fitted coaxially within a circular waveguide 30. In Figure 4 the face of the anode plate is shown and in Figure 5 an end of the anode plate is shown; in both gures magnetic coupling between the resonator remote from the choke cup and the waveguide is indicated by the dotted lines 31, and the transmission of energy along the waveguide is in the direction of the arrow 32.

The strength of the coupling of the magnetron to the waveguide may be controlled, in the design of the device,

lyvaitio of the dimensions marked x and e, rdtion of either, or both, increasing the coupling.

Figure 6 illustrates in a similar way a modification of the-magnetron described with reference to Figure 3, arranged for the excitation of a rectangular section waveguide; the modication consists of the rotation of the anode system through and of the addition of a second choke cup 33 at the opposite side of the anode of the choke cup 6 (in other modifications the choke cup 33 may be omitted). The waveguide system consists of a rectangular section waveguide intersected at right angles by a circular section waveguide 34, so as to leave on one side of the circular waveguide an open end rectangular section stub 35, the other rectangular section branch 36 providing the load coupling waveguide. The magnetron 37 is inserted coaxially along the circular section waveguide so that its anode system lies at the intersection of the two waveguides; coupling takes place from each resonator of the anode system to the rectangular waveguide sections 35 and 36; in the stub 35 is iitted a tuningV plunger 38, and the output energy of the magnetron 'is arranged to be transmitted along the branch 36 in the direction of the arrow 39 to a load represented by the rectangle 40, which may, yfor example, -be a mixer unit in a radio or radar receiver.

Byadjustment of the plunger 38 the reactive load on. the anode system of the magnetron 37 can be varied andi itsfrequency of oscillation controlled over an appreciable: range.

It will be appreciated that whilst in a magnetron in accordance with the invention the anode must be generally of plate form, the major faces need not everywhere be parallel, and in particular the shape of the anode segments may be varied for enabling the electrical constants of the anode system, such as the L/C ratio of the resonators, to be varied, for example for adjustment of the tuning range or for control of the output coupling. Thus Figures 7 and 8 illustrate two such modiiications in a magnetron of the kind shown in Figure 6, having only Va single choke cup.

Each of Figures 7 and 8 shows a section through the anode plate 41 mounted on a choke cup 42, the section being taken in a plane perpendicular to the major faces of the plate through the centres of the anode segments. In Figure 7 the thickness of each of the segments 43 tapers from the electron orbit space to the edge of the plate, whilst in Figure 8 the length of the anode at the electric orbit space is effectively increased by the addition 'of lips 46 at the end of the segments 45.

It may be observed that whilst the magnetrons in accordance with the invention have been described relative to their operation in the wmode, for which they are particularly designed, it will in general be possible for them to be operated in other modes, and this may sometimes-be desirable for obtaining different output frequencies.

We claim:

11.'A travelling wave magnetron oscillator suitable for operation on a space harmonic of the -n-mode of oscilla- 60"' tion of the anode resonator system, having a sealed evacuated envelope which is at least partly of insulating material and contains an anode in the form of a metal plate pierced perpendicular to its end faces by an aperture whose perimeter provides a cylindrical inner space which forms the electron orbit space, and an even plurality, not exceeding six, of spaces which form similar cavity resonators extending radially from the electron orbit space and arranged around its axis in harmonic positions, having a cylindrical cathode mounted coaxially within the electron orbit space, and having leads to the anode and cathode extending through the sealed envelope, the anode plate aperture being dimensioned to produce a 1r-mode resonant frequency corresponding to a free-space wavelength ?t with the diameter of the electron orbit space being not greater than M10 and its axial length being not, greater than SM10, whereinatleast alfree. @dan part of the anode plate about aline ot symmetry 0f;- one,

of the cavity'resonators isv positioned so close to an insulating materialpart :of the envelope that` the elettrt'i-t magnetic eld linked with said edge part will pass directly through said insulatingtmaterial part for the output coupling of the magnetronto a load waveguide.

2. A'travellingy wave magnetron oscillator according to claim 1, wherein the axial length of the electron orbit space is also not greater than M10.

3. A travelling wave magnetronVV oscillator according to claim l designed specic'ally for continuous wave operation and wherein the ratio of cathode diameter to the` diameter ofthe electron orbit space is about 0.3.

4. A travelling wave magnetron oscillator accordingto claim 1,'wherein the anode cavity resonators are arranged 'symmetrically around -the axis of the electron orbit space,

5. A travelling wave lmagnetron oscillator according to claim 1, wherein the angulanwidth of the mouth of the cavityresonators exceedsV 1r/,2k radians butis different.

from 21r`/k radians, where k is the valueot` Np/Zcorref Vspending tothe operating space harmonic, Nbcing the number of cavity resonatorsrand pbeing a small integer. 6. A travelling wave magnetron oscillator according Y to claim 1, wherein the anodef plate is mounted on the,

base of a Vcrzylindrical'choke cup within a coaxially cylindrical glass envelope, thesides of the choke. cup extend-y irig away from the anode, bein'gof axial length electrically, equivalent to M4, and approaching close to the inner.

surface of the envelope so as to forma quarter wavelengthich'oke when the envelope is insertdfin a closely tting waveguide. Y

`7. A travelling wave magnetron oscillatoraccordingto claim l, having four cavity resonatorsdening between themselves four rectangular anodeI segments, Yeachof which segments extendsY radially from the surface of the.v

electron orbit space with the segments Vsytrmgletrically arranged around the space.V l Y VV8. `A'travellin'g wave magnetron oscillator according. to claim 1, having two cavity resonatorsn each in' the shape of a segment of atcirclelcommunieating Withthc electron Vorbit space through a slot extending perpen- V dicular to and from the centre of the chord v'of the seg-v meiitQsaid chords being parallel to earch other. Y9. A travelling wave magnetron oscillator according.

to claim 8, wherein each of the said slots is parallel-sided near the electron orbit space and then flares outwards, with straightsides, to the segmental 'part of theresonator. i y Y l0. A travelling wave 'magnetron oscillator Vaccording to claim 8, wherein the Vanode plate is mounted on the'` baseof'a cylindrical choke cup within acoaxially cylindrical glass envelope with the chords of the segmental parts of the cavity resonators parallel to the base of the choke cup, the sides of the choke cupL extending away` from the anode and being of axial length electrically equivalent to M4, and wherein the curved sideoftheV segmental part of the cavityresonator further from the Y 12 loop` the electromagnetic eld linked-with which extends tothev exteriorof the envelope. Y

1,1,V A travellingwaye magnetron oscillator according to claim 8, whereinthe anode Vplate is mountedV between two, coaxiall cylindrical choke Vcups withinY a coaxiallyl cylindrical glass envelope, each choke cup Vextending -away from the" anode and being ofV axial length electrically equivalent to V4, wherein the chords of the segmental-parts of the cavity resonators extendV parallel to the axes of Vsaid choke cups and the curved side of eachsaid segmental part extends so close'tojthe edge of the anodeplate'that saidedge part ofiV the plate constitutes in eiect an output loop the electromagnetic ield linked with which extends to the'exterior of the envelope.

' 12. A travelling wave magnetron-oscillator according the'electron orbit space towards the edge of the plate.y

13.,A traveling Wave magnetron oscillator according Y to claim 8,V wherein the axial length of the anode at the,V electron orbit space isV increased by lips extending coaxial with the electron orbit space from the inner endV of theV anode segments, Y

Y 14. Thecombination .of a travelling wave magnetron! .A according-tto1claim,5-with a waveguide'wherein the cylindrical glass envelope is positioned coaxially within a closely fitting cylindrical waveguide so that outputenergy from themagnetron can excite oscillations in the waveguide. f

l5. The combination of a travelling wave magnetron oscillator. accordingy to claim 1l with crossed`cylindrical`Y chokejcup extendslso close to theedge of the plate that said'edgepart of the plate constitutesin effect an output waveguides whereinthe cylindrical'envelope of the magnetron is positioned coaxially within a rst one of said waveguides so that the anode liesat the intersection of the waveguideswith each branch of said rst waveguide extending from the intersection electrically closed byone of the magnetron choke cups, a load connected toone ofthe branchesv of the second waveguide for receiving oscillations excited in said` branch by the magnetron, and a,V plunger closing the other branch of said secondl waveguide, theposition oftwhich plungerV is adjustable for controlling the outputfrequency of the magnetron.

References Cited in the le of this patent VUNITED STATES PATENTS 2,063,342Y Samuel Dec. 8, 1936 2,209,923A Kilgore July 30, 1940'- 2,247,0 77 Blewett et al. June 24, 1941 2,418,469 Hagstrum Apr. 8, 1947 2,421,912 'Spooner lune 10, 1947 2,465,211 1 Donal et. a1 Mar. 22, 1949 27,477,122 n 'Garner July 26, 1949 2,482,495 Laidig Sept.- 20, 1949 2,616,063 Willshaw Oct. 28,l 1952v 2,4639g4t13Y4 Crout May 19, 1953 2,666,869 V"Clogston etal. `lan. 19, 1954 2,683,238 .Millman Julyr6, 1954.

2,775,721l Dench Dec. 25, 1956- Karp oct. 2s, V195sV 

